Diesel engines provide lower emissions and increased fuel economy compared to gasoline engines; however, environmental hazards may be posed by diesel exhaust emissions. Diesel particulate filters are now being deployed to control particulate emissions from diesel-powered equipment such as trucks, buses, diesel electric locomotives and generators. In particular, diesel particulate filters control particulate emissions by physically trapping soot particles in their structure. Diesel particulate filters are preferably constructed as wall-flow monoliths, which allow exhaust gases to flow through their porous ceramic walls, while any particulate present in the exhaust gas is collected on the upstream side of the wall. Once a predetermined condition is met, the filter may be cleaned by a regeneration cycle, during which the temperature of the exhaust gas is high enough to ignite and burn out any particulate soot. This regeneration cycle reduces the backpressure of the diesel particulate filter near to that of a new filter. The surface of the walls or the porous interior of the walls may include a catalyst wash coat containing platinum (Pt), palladium (Pd), iron (Fe), strontium (Sr) or rare earth elements such as cerium (Ce) to lower the temperature required for regeneration of the filter and to convert hydrocarbons and carbon monoxide in the exhaust gases into non-hazardous water vapor and carbon dioxide.
One preferred material for the manufacture of high temperature components is cordierite (Mg2Al4Si5O18), a magnesium aluminum silicate, that often includes low levels of iron or other impurities. Cordierite is a desired material for the manufacture of diesel particulate filters and other high temperature articles, such as catalytic converters, NOx adsorber substrates, catalyst substrates, and honeycomb articles because of its relatively low cost. Cordierite materials are typically manufactured by mixing a raw batch that includes talc, alumina, aluminum hydroxide, kaolin and silica. The batch is then blended with a binder (such as methylcellulose) and a lubricant (such as sodium stearate) to form a plastic mixture. This plastic mixture is then formed into a green body and sintered. U.S. Pat. No. 6,864,198 (hereby incorporated by reference in its entirety) assigned to Corning Incorporated discloses one method of forming a cordierite honeycomb structure.
During the regeneration of a cordierite DPF, heating of the core region creates a large thermal gradient across the radius of the filter, such that, for example, the core may be at 800° C. to 1000° C. while the periphery near the skin is at 400° C. to 500° C. Due to the positive coefficient of thermal expansion, CTE, of cordierite from 400 to 1000° C., the filter's core expands in the axial direction more than does the filter's outer shell. Consequently, strain is created between the skin and the core such that the core is in compression and the skin is in tension. If the stresses are sufficiently large, the skin may fracture, creating one or more cracks than encircle part or all the filter's circumference, orthogonal to the axial direction. Likewise, axial thermal gradients along the length of the filter may result in the face of the filter (inlet or outlet end) having a lower temperature than the interior of the filter. In such cases the face will be in tension, and face-cracking may occur. Such fractures of either type may compromise the filter's mechanical integrity, or filtration efficiency. Accordingly, cordierite filter articles which have better resistance to the formation of such cracks are highly desired.
Thermal stresses can be reduced by regenerating the filter more frequently before large amounts of soot can accumulate, thereby reducing both the maximum internal temperature and thermal gradient. However, increasing the frequency with which energy is supplied to raise the temperature of the exhaust gas entering the filter and, thus, initiate regeneration, results in lower fuel economy. Thus, it would be highly desirable to increase the filter's intrinsic thermal stress resistance, so that regeneration cycles may occur less frequently.
Thus, cordierite honeycomb articles having increased strength for greater mechanical durability, in combination with higher thermal shock resistance are much sought after. Such increases in strength and thermal shock resistance cannot, however, come at the expense of other important properties, such as clean pressure drop, and filtration efficiency, for example. Accordingly, there is a need for cordierite articles which exhibit improved strength and durability in regards to exposure to thermal cycles.